1. Field of the Invention
The invention in the field of biochemistry and immunology relates to novel peptides that induce and stabilize an immunogenic conformation in HIV-1 gp120 protein, and thereby provide compositions useful in a method for preventing or treating HIV infection.
2. Description of the Background Art
The failure of the VaxGen vaccine trial has convincingly dismissed the notion that simple recombinant monomeric HIV-1 gp120 proteins are sufficiently effective immunogens for AIDS vaccines. A clear consensus exists that induction of neutralizing antibodies will be an important feature of a successful AIDS vaccine, and that the most effective immunogen able to elicit these antibodies will have to contain some form of gp120. See, for example, Burton D R, 1997, Proc Natl Acad Sci USA. 94:10018-23; Burton D R, 2002, Nat Rev Immunol. 2:706-13; Zolla-Pazner S, 2004, Nat Rev Immunol. 4:199-210).
Therefore, the goal of many laboratories has been to produce more effective presentations of gp120 that could selectively induce the production of neutralizing antibodies. HIV gp120 can assume a conformation that reveals or stabilizes distinct epitopes that are able to elicit cross reactive neutralizing antibodies, and as such, represents a preferred presentation of the viral envelope to be used in a commercially useful prophylactic AIDS vaccine. Such a conformation of the viral envelope is induced when the envelope is complexed with CD4 (Thali, M et al., 1993, J Virol 67:3978-88; Kang, C Y et al., 1994, J Virol 68:5854-62; Sullivan, N et al., 1998, J Virol 72:4694-703; Xiang, S et al., 2003, Virology 315:124-34; DeVico, A L et al., 1995, Virology 211:583-8; Gershoni, J M et al., 1993, FASEB J 7:1185-7) leading to epitopes that have been referred to as CD4 induced (“CD4i”) epitopes.
The problem, however, is that association of gp120 with CD4, by definition, occupies and thereby obscures, the CD4 binding-site (CD4BS) of the virus, forfeiting a highly effective neutralizing epitope. Moreover, efforts to induce CD4i in the absence of CD4, e.g., by mutagenesis (Xiang, S H et al., 2002, J Virol 76:9888-99) came at the expense of losing credible neutralizing epitopes such as that of mAb b12. This led some investigators to conclude that the conformation of CD4i and CD4BS are mutually exclusive.
Since a study published in 1990 (Celada, F et al., 1990, J Exp Med 172:1143-50), there has been continuous growing interest in the CD4/gp120 complex as an immunogen. Originally, the intent was to identify novel sites on CD4 possibly involved in viral entry (Celada et al., supra) but by 1993 it was appreciated that the viral receptor might be able to lock gp120 into a preferred conformation, thereby revealing novel epitopes that have since become known as CD4i epitopes of the envelope. See also J. M. Gershoni, Int'l Pat. Publ., WO9415638A1, published Jul. 21, 1994.
The CD4i conformation of gp120 is operationally defined by the binding of mAbs that show a distinct preference for CD4-complexed gp120 as compared to free gp120. Examples of such defining antibodies are the murine mAbs CG10 (Gershoni et al., 1993, supra; Gershoni, 1994, supra) and 8F101 (DeVico et al., supra) and the human mAbs 17b, 48d (Thali et al., supra) and E51 (Xiang et al., 2003, supra). CD4i epitopes have been equated with the co-receptor binding site on gp120 (Xiang, S H et al.,2002, AIDS Res Hum Retroviruses 18:1207-17) and, as such, are perceived as potential vaccine candidates. This is based on the idea that CD4i epitopes represent relatively conserved epitopes of gp120 that become better exposed or immunogenic when the virus binds to CD4.
Subsequently, chemically cross linked versions of CD4/gp120 were shown to elicit neutralizing antibodies in goats and monkeys (Devico, A et al., 1996, Virology 218:258-63; Fouts, T et al., 2002, Proc Natl Acad Sci USA 99:11842-7). There is disagreement as to whether the neutralizing activity is actually due to anti-gp120 or rather to anti-CD4 antibodies (Varadarajan, R et al., 2005, J Virol 79, 1713-23).
If it were possible to replace CD4 with an alternative mimetic, then issues of anti-CD4 autoimmunity would be circumvented. To this end, modified scorpion toxins were examined and found to function as alternative molecules that could, upon binding to gp120, induce the CD4i conformation (Vita, C et al., 1999, Proc Natl Acad Sci USA 96:13091-6; Huang, C C et al., 2005, Structure 13:755-68). DNA encoding either D1-D2 domains of CD4 or the scorpion toxin mimetics were spliced with gp120-encoding DNA to create a single chain gene that encodes a protein with two components of the complex folded in a manner that allows them to bind to each other (Fouts, T R et al., 2000, J Virol 74:11427-36; He, Y., D'Agostino, P et al., 2003, Vaccine 21:4421-9).
Several other approaches to stabilize gp120 in a preferred conformation have been devised. Three possibilities for this are                (1) modifying the repertoire of sugar moieties of the glycoprotein to accentuate the desired accessible surfaces of gp120 (Pantophlet, R et al., 2003, J Virol 77:5889-901),        (2) complexing gp120 with unique antibodies that induce CD4i conformation (Liao, H X et al., 2004, J Virol 78:5270-8; Wyatt, R et al., 1995, J Virol 69:5723-33); or        (3) introducing strategically placed mutations to fix gp120 into the desired configuration (Xiang, S H, 2002, supra; Yang, X et al., 2000, J Virol 74:4746-54).        
A case in point for the latter approach was the introduction of the mutation 375 S/W in gp120 with the intention of locking gp120 into the CD4i conformation, postulating that a Trp residue at position 375 would effectively simulate the Phe at position 43 of CD4. Whereas this mutation induced the 17b epitope in gp120 (i.e., induced binding to mAb 17b, a “marker” of the CD4i conformation), the structure simultaneously lost the b12 epitope which overlaps the CD4BS. It was concluded that the CD4BS conformation (represented by an intact b12 epitope) and the CD4i conformation (represented by 17b binding) are distinct, different and mutually exclusive.
MAb A32 has been used to induce CD4i, and this complex was tested as an immunogen (Liao et al., supra). Whereas A32 complexed with gp12089.6 produced more neutralizing antibodies than gp12089.6 alone, this property was not found for the A32/gp120BaL complex. It was concluded that monomeric gp120BaL, rather than a complex, was the best immunogen of those tested.
Therefore, a well-recognized need remains to overcome the deficits in the prior art described above. The present invention is intended to provide novel solutions to such problems.
Citation of the above documents is not intended as an admission that any of the foregoing is pertinent prior art. All statements as to the date or representation as to the contents of these documents is based on the information available to the applicant and does not constitute any admission as to the correctness of the dates or contents of these documents.